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September, 09/03/2015
Events and times subject to change

September 3, 2015 Thursday 12:30 PM  +
Meyer 6th Floor CSMR Area
Soft Condensed Matter Seminars (csmr)


Ana Lisica
Max Planck Institute Dresden

Single-molecule Studies of Transcription by RNA Polymerases I and II

Transcription elongation is often interrupted by RNA polymerases (Pol) backtracking. To recover from the backtracked state, Pol I possesses a strong intrinsic transcript cleavage activity, while that of Pol II is weak but can be enhanced by transcription factor TFIIS. However, backtrack recovery can also proceed by 1D diffusion, and the mechanisms that underlie the choice of recovering by either 1D diffusion or cleavage remain unclear. Here, we used high-resolution dual-trap optical tweezers to compare Pol I and Pol II transcription and backtracking dynamics. We found that individual molecules of Pol I transcribed faster than those of Pol II, paused less often, and transcribed against higher opposing forces. On their own neither enzyme could recover from backtracks beyond a characteristic depth, and only TFIIS-dependent cleavage allowed Pol II to rapidly recover from backtracks of arbitrary depth. Using stochastic theory we quantified distinct diffusion and cleavage rates of Pol I and Pol II and showed that the choice of a backtrack recovery pathway is determined by a kinetic competition between 1D diffusion and transcript cleavage. Taken together, our work identified distinct backtrack recovery strategies of Pol I and Pol II, shedding light on the evolution of cellular functions of these key enzymes.


September 3, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


M. Zahid Hasan
Princeton University

Discovery of Weyl Fermion and Topological Fermi Arc Quasiparticles in Condensed Matter Systems

Topological matter can host Dirac, Majorana and Weyl fermions as quasiparticle modes on their boundaries. First, I briefly review the basic concepts defining insulators and superconductors where topological surface state (Dirac and Majorana) modes are robust only in the presence of a gap (Hasan and Kane; Rev. of Mod. Phys. 82, 3045 (2010)). In these systems topological protection is lost once the gap is closed turning the system into a trivial metal. A Weyl semimetal is the rare exception in this classification scheme which is a topologically robust metal (semimetal) whose low energy excitations are Weyl fermion quasiparticles. In a Weyl semimetal, the chiralities associated with the Weyl fermion nodes can be understood as topological charges, leading to split monopoles and anti-monopoles of Berry curvature in momentum space. This gives a measure of the topological strength of the system. Due to this topology a Weyl semimetal is expected to exhibit 2D Fermi arc quasiparticles on its surface. These arcs (“fractional” Fermi surfaces) are discontinuous or disjoint segments of a two dimensional Fermi contour, which are terminated onto the projections of the Weyl fermion nodes on the surface (Xu, Belopolski et.al., Science 349, 613 (2015)). I show that Fermi arc quasiparticles can only live on the boundary of a 3D crystal which collectively represents the realization of a new state of quantum matter.


September 4, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Michael Strauss
Princeton

LSST (Final Title TBA)



September 9, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor Conference Room
Soft Condensed Matter Seminars (csmr)


Shenshen Wang
MIT

TBA



September 9, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Gabrijela Zaharijas
University of Nova Gorica

The nature of GeV Galactic Center excess emission: options and their testability

Several groups have recently claimed evidence for an unaccounted gamma-ray excess over the diffuse backgrounds at few GeV in the Fermi-LAT data in a region around the Galactic Center, consistent with putative signals of long sought for WIMP dark matter particles. However, several conventional astrophysical explanation of this emission appear viable and need to be understood and accounted for, before any robust inference can be made about dark matter signals. In particular, we show that the main features of this excess can be reproduced if they originate in the inverse Compton emission from high-energy electrons injected in a burst event of ~10^52-10^53 erg roughly O(10^6) years ago and we discuss the testability of this prediction. Another convincing astrophysical candidate for this emission is an unresolved population of MSP. There have been several works studying this possibility and reaching in some cases opposite conclusions. Here we reassess these claims by adopting a phenomenological approach and make clear predictions for the next generation experiments, which due to their improved angular resolution at few GeV should be able to test the point source origin of the excess.


September 9, 2015 Wednesday 2:00 PM  +
Meyer 611
Soft Condensed Matter Seminars (csmr)


Shenshen Wang
MIT

Guiding Antibody Evolution to Catch the Virus

The adaptive immune system houses amazingly efficient evolutionary processes to protect higher organisms from diverse invading agents. A key component of effective immune responses is the generation of potent antibodies by a real-time Darwinian process called affinity maturation. However, devastating circumstances are raised by rapidly mutating viruses (e.g. HIV which infects the immune system itself) that can escape antibody recognition. None of the classical vaccination strategies has succeeded to date.
In this talk, I will present an in silico model of affinity maturation driven by multiple antigen variants, a basic problem in cell biology that has not been considered before. I will show that the understanding obtained, using concepts from condensed matter physics, not only explains why the emergence of cross-reactive antibodies is so rare upon natural infection, but also suggests novel strategies to guide their evolution. The proposed vaccination scheme has been shown in mouse experiment to be very effective at inducing cross-reactive antibodies focused on the vulnerable part of the virus.


September 11, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Frank Bosch
Yale

Coming of Age in the Dark Sector

The assembly of dark matter halos is a highly non-linear process that is typically addressed using N-body simulations. However, since the initial conditions are Gaussian, and gravity is scale-free, it is also possible to construct simple analytical models that capture the essentials of structure formation in a CDM cosmology. I present a new, analytical model for the assembly of dark matter halos that can be used to predict their structure, their substructure population, and their detailed assembly history. I show that the model is as accurate as N-body simulations, but with the advantage that it is orders of magnitude faster, and gives powerful insights. I present several applications, including a statistical assessment of the "Too-Big-To-Fail" problem, a demonstration that the growth history of dark matter potential wells is universal, and a discussion of subhalo segregation.


September 14, 2015 Monday 2:00 PM  - September 15, 2015 Tuesday 3:00 PM +
Meyer 6th Floor
Other Physics Department Events (other)

International French-US Workshop on Nanoelectronics

A very successful Partner University Fund (PUF) project involving leading research laboratories in the US (University of California San Diego, New York University) and France (University of Paris Sud and University of Lorraine) has resulted in a range of high-impact outcomes. These include cofunded US-French projects, co-organized workshops, regular faculty visits between laboratories, student exchanges, several high-profile publications and meetings focused on establishing joint or dual degrees. The PUF project has also been essential to promote the creation of an International Associated Laboratory (LIA CNRS) on Nanoelectronics that started in January 2015.

In the framework of this very fruitful US-French collaboration, the aim of this workshop is to gather the scientific and engineering communities in both countries in the fast growing field of Nanoelectronics including, in particular, novel ultra-low power charge and spin based nanodevices and bio-inspired circuits. It will provide attendees opportunities to discuss new developments and to initiate and reinforce French-US collaborations and research projects. The workshop will benefit from the attendance of well-known scientists from academy, industry, and startup companies. A special event celebrating the successful collaboration between US and French partners will be held at the French embassy on September 15th.

- PROGRAM -

Workshop Program in PDF



September 16, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor CSMR Area
Soft Condensed Matter Seminars (csmr)


Baruch Meerson
Racah Institute of Physics, Hebrew University of Jerusalem

Extinction of established populations: A physicist's view

Extinction of a long-lived population can result from a large fluctuation coming from the discreteness of individuals and stochasticity of their births, deaths and interactions. I will show how one can use WKB approximation to evaluate the mean time to extinction. In the WKB framework the most likely path of the population to extinction is encoded in an instanton-like trajectory of an underlying classical Hamiltonian system.


September 16, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Mehrdad Mirbabayi
IAS

TBD



September 16, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor Conference Room
Soft Condensed Matter Seminars (csmr)


Baruch Meerson
Racah Institute of Physics, Hebrew University of Jerusalem

Extinction of Established Populations: A Physicist's View

Extinction of a long-lived population can result from a large fluctuation coming from the discreteness of individuals and stochasticity of their births, deaths and interactions. I will show how one can use WKB approximation to evaluate the mean time to extinction. In the WKB framework the most likely path of the population to extinction is encoded in an instanton-like trajectory of an underlying classical Hamiltonian system.


September 17, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Cliff Brangwynne
Princeton University

Measuring the Intracellular Dew Point: Phase Transitions in Cells

Increasing evidence suggests that phase transitions play an important role in the internal organization of living cells. We have shown that a number of membrane-less RNA and protein rich organelles, known as RNP bodies, represent condensed liquid phase droplets, which assemble by liquid-liquid phase separation. The nucleolus is a liquid-like nuclear body, which plays an important role in cell growth and size homeostasis. Using the reductive cell divisions of early C.elegans embryos, we show that a simple phase threshold model explains striking features of the intrinsic coupling of nucleolar assembly to cell size. Building on these findings, we develop a Cahn-Hilliard model for nucleolar liquid-liquid phase separation, which can quantitatively account for the dynamics of nucleolar assembly, for both an in vitro reconstituted system, and within living cells. We use a custom microfluidics platform to test these models in growing C.elegans worms, where we find that the nucleolus grows proportional to cell and organism growth. Concentration-dependent phase transitions allow the cell to read-out its size, and could provide a novel biophysical feedback mechanism for cell growth control.


September 18, 2015 Friday 11:00 AM  +
Meyer 5th Fl. CCPP Lounge
Other CCPP (ccpp)

Informal Astro Talk
Matteo Biagetti
University of Geneva

TBA



September 23, 2015 Wednesday 11:00 AM  +
Meyer 611
Hard Condensed Matter Seminars (hcmp)


Karin Everschor
University of Texas at Austin

Interaction Correction to the Magneto-Electric Polarizability of Z_2 Topological Insulators

When time-reversal symmetry is weakly broken and interactions are neglected, the surface states of a Z_2 topological insulator support a half-quantized Hall conductivity. A surface state Hall conductivity in an insulator is equivalent to a bulk magneto-electric polarizability. By performing an explicit calculation for the case in which the surface is approximated by a two-dimensional massive Dirac model and time-reversal symmetry is broken by weak ferromagnetism in the bulk, we demonstrate that there is a non-universal interaction correction to the surface Hall conductivity. For thin films interaction corrections to the top and bottom surface Hall conductivities cancel, however, implying that there is no correction to the quantized anomalous Hall effect in magnetically doped topologically insulators.


September 23, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor Conference Room
Soft Condensed Matter Seminars (csmr)


Sanghyuk Lee
Rutgers University

TBA



September 23, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Eva Silverstein
Stanford University

Long-range non-locality in six-point string scattering



September 24, 2015 Thursday 11:00 AM  +
Meyer 611
Hard Condensed Matter Seminars (hcmp)


Matthias Sitte

Tight-Binding Approach towards an Effective Model for InAs/GaSb Quantum Wells

Topological insulators have attracted a great deal of attention as a new quantum state of matter in the last decade. The first realizations of 2D TIs were HgTe/CdTe quantum well heterostructures, but in recent years another class of semiconductor heterostructures --- namely InAs/GaSb quantum wells --- was shown to yield 2D TIs as well. Compared to the HgTe/CdTe-based systems they have many advantages, most prominently a continuously tunable band structure via external electric fields and stronger proximity coupling to superconductors. We perform empirical tight-binding calculations on these systems to study how topological properties are changed by varying external control parameters such as electric fields or well thicknesses.


September 25, 2015 Friday 12:45 PM  +
Meyer 611
Other Physics Department Events (other)

Mini Symposium

12:45 PM Chair - Opening Remarks
1:00 PM Talk 1 – Kyle Cranmer
1:30 PM Talk 2 - David Pine
2:00 PM Talk 3 – Mark Gershow/Dima Rinberg (TBD)
2:30 PM Talk 4 – Aditi Mitra
3:00 PM Talk 5 – Michael Blanton
Break
3:15 PM Talk 6 – Alexandra Zidovska
3:45 PM Talk 7 – Andrew Kent/Andrew Wray
4:15 PM Talk 9 – Matthew Kleban
5:00 PM Happy Hour


September 25, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Roman Rafikov
IAS

TBA



September 30, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor Conference Room
Soft Condensed Matter Seminars (csmr)


Roseanna Zia
Cornell University

A Micro-Mechanical Study of Coarsening and Rheology of Colloidal Gels:
Cage Building, Cage Hopping and Smoluchowski's Ratchet

Reconfigurable soft solids such as viscoelastic gels have emerged in the past decade as a promising material in numerous applications ranging from engineered tissue to drug delivery to injectable sensors. These include colloidal gels, which microscopically comprise a scaffoldlike network of interconnected particles embedded in a solvent. Network bonds can be permanent or reversible, depending on the nature and strength of interparticle attractions. When attractions are on the order of just a few kT, bonds easily rupture and reform. On a macroscopic scale, bond reversibility allows a gel to transition from solidlike behavior during storage, to liquidlike behavior during flow (e.g., injection or shear), and back to solidlike behavior in situ. On a microscopic scale, thermal fluctuations of the solvent are occasionally strong enough to break colloidal bonds, temporarily allowing particles to migrate and exchange neighbors before rebonding to the network, leading to structural evolution over time. Prior studies of colloidal gels have examined evolution of length scales and dynamics such as decorrelation times. Left open were additional questions such as how the particle-rich regions are structured (liquidlike, glassy, crystalline), how restructuring takes place (via bulk diffusion, surface migration, coalescence of large structures), and the impact of the evolution on rheology. In this talk I discuss these themes as explored in our recent dynamic simulations. We find that the network strands comprise a glassy, immobile interior near random-close packing, enclosed by a liquidlike surface along which the diffusive migration of particles drives structural coarsening. We show that coarsening is a three-step process of cage forming, cage hopping, and cage arrest, where particles migrate to ever-deeper energy wells via “Smoluchowski’s ratchet.” Both elastic and viscous high-frequency moduli are found to scale with the square-root of the frequency, similar to the perfectly viscoelastic behavior of non-hydrodynamically interacting, purely repulsive dispersions. But here, the behavior is elastic over all frequencies, with a quantitative offset between elastic and viscous moduli, which owes its origin to the hindrance of diffusion by particle attractions. Propagation of this elasticity via the network gives rise to age-stiffening as the gel coarsens. This simple phenomenological model suggests a rescaling of the moduli on dominant network length scale that collapses moduli for all ages onto a single curve. We propose a Rouse-like theoretical model and, from it, derive an analytical expression that predicts the effects of structural aging on rheology whereby linear response can be determined at any age by measurement of dominant network length scale—or vice versa.
Extending these studies to the non-linear response regime, we examined the transition from solid-like to liquid-like behavior, a yielding process that is not instantaneous but rather occurs after a finite delay. Experimental studies have shown that the delay time decreases as stress increases, but the underlying microstructural origin is not clear. Recent experiments reveal two stress regimes, suggesting multiple yield mechanisms. Theories advanced to link gel structure to rheology aim to predict the long-time response of a gel under an applied load. While these hypothesize a competition between bond breakage and reconnection rates, no such particle-scale dynamics have been directly observed, and it is not clear these theories reconcile with ongoing structural evolution. To study these behaviors, we conduct large-scale dynamic simulation to model structural evolution and particle transport in colloidal gels subjected to a step stress. We recover the finite delay-time preceding yield, and also discover a new regime whereby intermediate values of the stress cause the gel to yield, flow, but eventually re-solidify. From this we develop a phase map to predict the nonlinear response of a reversible gel to any imposed step stress. A range of volume fraction, attraction strength, and stress is studied, with detailed connection between macroscopic response, microstructure, and particle dynamics.


September 30, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Douglas Stanford
IAS

TBD



October 1, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


John Preskill
Caltech

TBA



October 2, 2015 Friday 11:00 AM  +
Meyer 5th Fl. CCPP Lounge
Other CCPP (ccpp)

Informal Astro Talk
Ting-Wen Lan
Johns Hopkins University

Diffuse Interstellar Bands or Mg II absorbers



October 2, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Tsvi Piran
Hebrew University

TBA



October 7, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Alberto Nicolis
Columbia University

TBA



October 8, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Daan Frenkel
University of Cambridge

TBA



October 9, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Rita Tojeiro
University of St Andrews

TBA



October 16, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Schuyler Dyk
IPAC/Caltech



October 21, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


V. Nair
City College of New York - CUNY

TBD



October 23, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Marusa Bradac
University of California - Davis

TBA



October 28, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor CSMR Area
Soft Condensed Matter Seminars (csmr)


Kinneret Keren
Technion

TBA



October 28, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Matthew Low
IAS

tba



October 30, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Melissa Graham
Berkeley

Type Ia Supernovae: Progenitor Constraints from Late-Time Observations



November 4, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Surjeet Rajendran
UC Berkeley

TBD



November 5, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Misha Lukin
Harvard University

TBA



November 6, 2015 Friday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Victoria Grinberg
MIT

TBA



November 11, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Stefania Gori
Perimeter Institute

TBD



November 11, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor Conference Room
Soft Condensed Matter Seminars (csmr)


Vincenzo Vitelli

TBA



November 12, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Nuh Gedik
MIT

TBA



November 18, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


Wolfgang Altmannshofer
Perimeter Institute

TBD



November 19, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Ludwik Leibler
CNRS

TBA



November 20, 2015 Friday 12:00 PM  +
Meyer 5th Fl. CCPP Lounge
Astrophysics and Relativity Seminars (astro)


Charlie Conroy
Harvard University



December 2, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


David Simmons-Duffin
IAS

TBA



December 3, 2015 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Nima Arkani-Hamed
IAS

TBA



December 4, 2015 Friday 2:30 PM  +
Columbia University
Other CCPP (ccpp)

Big Apple Colloquium
Tremaine/Phinney Tremaine/Phinney
IAS/Caltech

The Final Parsec Problem

Can two super-massive black holes in a galactic nucleus merge in a Hubble time? Mini-Symposium by Scott Tremaine (IAS) and Sterl Phinney (Caltech). Part I -- Theory: Phinney (15 minutes): galaxy merger rates and BH demography from cosmology. Tremaine (30 minutes): the physics of what happens in the final parsec. -- SHORT BREAK -- Part II -- Observations: Tremaine (15 minutes): optical searches for binary BHs through broad line shifts. Phinney (30 minutes): other electromagnetic observations of binary BHs and future observations of merging BHs via gravitational waves. Followed by wine & cheese.


December 9, 2015 Wednesday 2:00 PM  +
Meyer 6th Floor Conference Room
Soft Condensed Matter Seminars (csmr)


Itai Cohen
Cornell University

TBA



December 9, 2015 Wednesday 2:00 PM  +
Meyer 5th Fl. CCPP Lounge
High Energy Physics Seminars (hep)


David Kaplan
Johns Hopkins University

TBD



January 28, 2016 Thursday 4:00 PM  +
Meyer 122
Physics Colloquia (colloquia)


Tim Mitchison
Harvard University

TBA



April 21, 2016 Thursday 4:00 PM  +
Meyer 122
Soft Condensed Matter Seminars (csmr)


Ann Nelson
University of Washington

TBA